Potassium monitoring in whole blood is one of the most important routine analysis performed in clinical laboratory, it is of fundamental importance both for the early detection of post-operative shock and for heart surgery. Determination of potassium contents of serum, urine, and foods is also very important in clinical and medical fields, since the potassium contents are related to renal diseases. These diseases restrict patients to a diet containing a large amount of potassium. From the potassium determination, medical information concerning physical conditions of the patient can be obtained. In the case of hypopotassemia, alkalosis, cirrhosis of liver, diuretic drugs, etc. are suspected. On the other hand, when potassium concentration in human serum becomes higher than 9 mmol dm−3, heart often stops.
Biological active-transport systems involving ions, in particular K+, have important functions in the organism, which are essential for regulation of many intracellular activities. These systems are related in the transmission of information by the nervous system and in the excitation and relaxation cycle of muscle tissue. Principally, accurate, easy and rapid sensing of potassium ions in human blood serum is very important prior to cardiac surgery to assess the condition of the patient.
Crown ethers have been reported to be an inexpensive neutral carrier in the construction of Ion Selective Field Effect Transistors. Additionally, these carriers have the option of covalently attaching the desired molecule for electro analytical applications. The characteristic of crown ether is their selective complexation ability. They bind the cationic portion of alkali and alkaline earth metal salts (guest) in to the cavity of the crown ring (host). The selectivity is dependent principally on the relative size of the cavity of the crown ring and the diameter of the cation, number of donor atoms in the crown ring and the topological effect and the relationship between the hardness of the cation and that of the donor atom, and charge number of the cation. In the case of crown complexes, a metal cation-anion contact always occurs from the open faces of the ring plane. Dibenzo-18-crown-6 used here was shown to have a circular cavity of diameter 2.6-3.2 A° which fits the exact size of potassium ion of 2.66 A° and makes it an excellent choice to be a sensing material for potassium ions.
Potentiometry using ion selective electrodes (ISE) is the method of choice due to the easy and fast performance of the assay. About 200 million clinical assays of potassium every year are performed using ISEs in the USA. It is well-known that ion selective electrodes are based on the use of a water-insoluble membrane that can be considered as a water-immiscible liquid of high viscosity, containing an ionophore which binds selectively the ion of interest and it generates a membrane potential. Potentiometric detection based on ion-selective electrodes, as a simple method, offers several advantages such as speed and ease of preparation and procedures, simple instrumentation, relatively fast response, wide dynamic range, reasonable selectivity and low cost. Besides, they are ideally suitable for on-site analysis and, nowadays, were found to be applicable in the analysis of some biologically relevant ions, process control and environmental analysis. Miniaturization of the system is realized using silicon technology. The potassium concentration is measured potentiometrically using ISFET coated with crown ether in combination with an Ag/AgCl reference electrode integrated on the same chip. To overcome problems resulting from a long time contact of the sensor with protein-containing sample solution, an automated measuring protocol was applied where the sensor is brought in contact with the sample only for short time segments. Immediately after the stabilization of the measuring signal the chip is flushed with a commercial Ringer solution of constant potassium concentration. The frequency of sample/conditioning solution cycles depend on the diagnostic demand. In this way the active sensing area of the sensor is cleaned time to time from the sample. Furthermore, the sensor signal in the cleaning solution serves as a calibration point.
Potentiometric ion sensors based on ion-sensitive field-effect transistors (ISFETs) are attracting increasing attention primarily because of their small size, robustness, low cost, fast response time and low output impedance. For the ISFET, the metal connection of the reference electrode acts as a remote gate. The equation giving the dependence of threshold voltage on the pH of the solution in contact with the gate is,vTh(ISFET)=Eref−Φsi−ψ+χ−Qf/Cd+2|φp|+1/Cd√2∈o∈sqNA(2|Ψp|)  (1)where Eref is the constant reference electrode potential Φsi is the silicon work function, χ is surface dipole potential of the solvent and Ψ is the interfacial electrostatic potential at the solution/dielectric interface whose sensitivity to changes in bulk pH is expressed by the equation,∂Ψ/∂pH=−2.303(RT/F)α  (2)R is universal gas constant, T is absolute temperature, F is Faraday's constant and α is a dimensionless sensitivity parameter (0<α<1), given by,α=1/(2.303KTC/q2β)+1  (3)k is Boltzmann constant, T is temperature in Kelvin scale, C is the differential double layer capacitance at the insulator-electrolyte interface, q is electronic charge and β is surface proton buffer capacity determining the ability of the gate dielectric surface to absorb or release protons.
But the silicon dioxide-silicon nitride gate ISFET shows sensitivity to various ions in aqueous solution such as H+, Na+, K+, Ca+, Zn++, Fe++, etc., besides H+-ion. Extension of ISFETs for measuring species other than hydrogen ions is a vital research area. ISFETs with ion-sensitivity and selectivity to different ionic species can be fabricated by depositing polymeric membranes containing specific receptor molecules on the gate surface.
Reference to be made to a publication by Shoji Motomizu et al, Analyst, 1988, 113, 743-746 wherein potassium in river water was determined by a spectrophotometric method involving flow injection coupled with solvent extraction. Dibenzo-18-crown-6 was used along with the dichloro derivative of ethyl orange. The procedure was shown to have less interference from foreign ions and the sensitive up to a potassium ion concentration of 10−5 M. The main problem is that dye employed in this study was dissolved in lithium hydroxide and hence the pH of the reagent solution was 10 which were unsuitable to determine potassium ions in blood serum.
Reference may be made to a publication by E. Malavolti et al, Analytica Chimica Acta 1999, 401, 129-136 wherein an optrode for continuous monitoring of potassium in whole blood was realized using valinomycin as ionophore and a neutral chromoionophore whose absorbance depends on the pH of the local environment. Membrane preparation was complicated and involves the following chemicals: chromoionophore, tetrahydrofuran, valinomycin, potassium tetrakis(4-chlorophenyl)borate, bis(2-ethylhexyl) sebacate and PVC. The main drawback of this protocol is that the sensor is sensitive to pH and maintenance of the thickness of the membrane. Otherwise, as the potassium concentration changes, all the components in the membrane bulk would shift to the new equilibrium so that there will be a change in the signal.
Reference may be made to a publication by P. C Pandey and R. Prakash, Sensors and Actuators 1998, B 46, 61-65 wherein a potassium ion-selective electrode using PVC matrix membrane impregnated with dibenzo-18-crown-6 at the surface of the polyindole modified electrode is reported. The lowest detection limit for the potassium ion sensor is 7.0×10−6 mol dm−3. The inherent disadvantages of this work are that preparation of sensor electrode requires tedious and complicated procedure and the selectivity of potassium ion over other cations in the same solution or human blood serum is not reported. The lowest detection limit was higher than the concentration of potassium in blood serum.
Reference may be made to a publication by Albrecht Uhlig et al, Sensors and Actuators B, 1996, 34, 252-257 wherein a miniaturised ion-selective sensor chip for potassium measurement in vivo for whole blood. Here, Valinomycin was used as ionophore and the potassium concentration is measured potentiometrically using an ion selective polymer membrane in combination with an Ag/AgCl/p-HEMA reference electrode integrated on the same chip. The main problem is the clotting of blood that has to be prevented by addition of an external reagent and unacceptable drift, which might be due to the solid state internal contact, loss of membrane ingredients and water absorption.
Reference may be made to a publication by Johan Bobaka et al, Analytica Chimica Acta, 1999, 385, 195-202 wherein all-solid-state potentiometric potassium-selective electrodes with plasticizer-free membranes were prepared by incorporation of valinomycin as the ionophore and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the lipophilic additive in a semiconducting conjugated polymer matrix of poly(3-octylthiophene). The membrane components were dissolved in chloroform and deposited on glassy carbon by solution casting. The main bottleneck is the sub-Nernstian response, the degradation of the response with time, especially for thin membranes and the observation of a relatively large ion transfer resistance at the membrane/solution interface.
Reference to be made to a publication by Carlos Alexandre Borges Garcia et al, Journal of Pharmaceutical and Biomedical Analysis 2003, 31, 11-18 wherein a simple and rapid method was developed for the K+ions determination employing a flow injection system using a flow-through electrode based on the naturally-occurring antibiotic ionophore nonactin occluded in a polymeric membrane. The nonactin ionophore was trapped in poly (ethylene-co-vinyl acetate) (EVA) matrix (40% w/w in vinyl acetate) and dispersed on the surface of a graphite-epoxy tubular electrode. The plasticizer-free all-solid-state potassium-selective electrode showed a linear response for K+ ion concentrations between 5.0×10−5 and 5.0×10−2 M with a near-Nernstian slope of 51.5 mV per decade, when Tris-HCl buffer (pH 7.0; 0.1 M) was employed as a carrier. The major setback is the maintenance of neutral pH in samples with ammonium ion as analytically interfering ion and sensing range of K+ ion concentration was well above the blood serum range.
Reference to be made to a publication by N. Abramova et al, Talanta, 2000, 52, 533-538 wherein application of a potassium ion sensor based on an ion sensitive field effect transistor (ISFET) for ion control of a dialysis solution in an artificial kidney and in blood plasma of patients treated by hemodialysis is presented. Commercial potassium ionophore valinomycin is used. The studied ISFETs have the required stability and sensitivity to monitor the potassium ion concentration in dialysis solutions within the artificial kidney apparatus. The major drawback is that ISFETs have not been tried on real blood samples.
Reference to be made to a publication by Daniela P. A. Correia et al, Talanta, 2005, 67, 773-782 wherein an array of potentiometric sensors for simultaneous analysis of urea and potassium in blood serum samples were developed. Urea biosensors based on urease immobilized by crosslinking with BSA and glutaraldehyde coupled to ammonium ion-selective electrodes were included in arrays together with potassium, sodium and ammonium PVC membrane ion-selective electrodes. Coupling of biosensors with ion-selective electrodes in arrays of sensors raises a few problems related to the limited stability of response and unidirectional cross-talk of the biosensors, and this matter was also subjected to investigation in this work. Up to three identical urea biosensors were included in the arrays, and the data analysis procedure allowed the assessment of the relative performance of the sensors. The major disadvantage is lack of identical cross-talk between urea sensors with other biosensors. This arises mainly due to the irregular enzymatic layer in some biosensors as a consequence of the procedure used for enzyme immobilization.